Method for driving liquid crystal display device

ABSTRACT

An object is to suppress deterioration of a displayed image even when a refresh rate is reduced in displaying a still image. A liquid crystal display device includes a pixel transistor electrically connected to a pixel electrode, and a capacitor having one electrode electrically connected to the pixel electrode and the other electrode electrically connected to a capacitor line. The pixel transistor is turned on and a voltage based on an image signal is supplied to the pixel electrode, and then, the pixel transistor is turned off so that a holding period during which the pixel electrode holds the voltage based on the image signal starts. A holding signal corresponding to change of the voltage based on the image signal in the pixel electrode in the holding period is supplied to the capacitor line so that a potential of the pixel electrode is constant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/976,431, filed Dec. 22, 2010, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2009-295608 on Dec. 25, 2009, both of which are incorporated byreference.

TECHNICAL FIELD

The present invention relates to a method for driving a liquid crystaldisplay device, a liquid crystal display device, or an electronic deviceincluding a liquid crystal display device.

BACKGROUND ART

Liquid crystal display devices are widely used in large display devicessuch as television sets and small display devices such as mobile phones.Higher value-added devices have been demanded and the development hasprogressed. In recent years, attention is attracted to the developmentof low power consumption liquid crystal display devices, in terms of theincrease in interest in global environment and improvement inconvenience of mobile devices.

Non-Patent Document 1 discloses a structure in which the refresh rate inthe case of displaying a moving image and that in the case of displayinga still image are different from each other in order to reduce powerconsumption of a liquid crystal display device. Moreover, Non-PatentDocument 1 discloses a structure in which, in order to prevent flickersfrom being perceived with change in drain-common voltage due toswitching of signals in a break period and a scanning period when astill image is displayed, alternating-current signals with the samephase are applied to a signal line and a common electrode also in abreak period so that the drain-common voltage does not change.

REFERENCE

-   Non-Patent Document 1: Kazuhiko Tsuda, et al., “Ultra low power    consumption technologies for mobile TFT-LCDs”, IDW'02, pp. 295-298    (2002)

DISCLOSURE OF INVENTION

As in Non-Patent Document 1, lower power consumption can be realized bya reduction in refresh rate. However, a voltage between a pixelelectrode and a common electrode cannot be kept constant in some casesbecause the potential of the pixel electrode is changed by the off-statecurrent of a pixel transistor and/or leakage current from liquidcrystals. Therefore, a displayed image deteriorates because a voltageapplied to the liquid crystals is changed.

An object is described in detail, using a specific example shown indrawings. FIG. 14A is a schematic diagram of a display panel in a liquidcrystal display device. A display panel 1400 in FIG. 14A includes apixel portion 1401, a gate line (also referred to as a scan line) 1402,a signal line (also referred to as a data line) 1403, a pixel 1404, acommon electrode 1405, a capacitor line 1406, and a terminal portion1407.

FIG. 14B illustrates the pixel 1404 in FIG. 14A. The pixel 1404 includesa pixel transistor 1408, a liquid crystal element 1409, and a capacitor1410. A gate of the pixel transistor 1408 is connected to the gate line1402. A first terminal serving as one of a source and a drain of thepixel transistor 1408 is connected to the signal line 1403. A secondterminal serving as the other of the source and the drain of the pixeltransistor 1408 is connected to one electrode of the liquid crystalelement 1409 and a first electrode of the capacitor 1410. The otherelectrode of the liquid crystal element 1409 is connected to the commonelectrode 1405. A second electrode of the capacitor 1410 is connected tothe capacitor line 1406. Note that the pixel transistor 1408 is a thinfilm transistor (TFT) including a thin semiconductor layer.

FIG. 14C illustrates the pixel 1404 in a manner different from FIG. 14Bin order to focus on each of the wirings and the elements. The referencenumerals of the wirings and the elements are the same as those in FIG.14B. Note that for description of the liquid crystal element 1409, FIG.14C illustrates a pixel electrode 1411 as the electrode on the pixeltransistor 1408 side, a counter electrode 1412 as the electrode on thecommon electrode 1405 side, and a liquid crystal 1413 placed between thepixel electrode 1411 and the counter electrode 1412.

FIG. 15A illustrates the same diagram as FIG. 14C and focuses on apotential of each wiring, a voltage between electrodes, and a currentflowing through each element. An image signal supplied to the signalline 1403 is a voltage V_(data). A voltage held in the pixel electrode1411 is V_(pix). A voltage of the counter electrode 1412 is V_(com). Avoltage applied to the liquid crystal 1413 is V_(LC). FIG. 15A alsoillustrates an off-state current I_(TFT) of the pixel transistor and acurrent I_(LC) flowing through the liquid crystal.

FIG. 15B is a general timing chart showing the potential of each wiringand the voltage between electrodes illustrated in FIG. 15A. In thetiming chart in FIG. 15B, a period is divided into periods F1 to F4. Thefollowing description is made on the assumption that the same image,that is, a still image is displayed in the periods F1 to F4. That is, inthe periods F1 to F4, the voltage V_(LC) applied to the liquid crystal1413 is a constant voltage V_(data) (indicated by an arrow 1501 in FIG.15B). It is known that the liquid crystal element 1409 deteriorates byapplication of voltage in one direction to the liquid crystal 1413, andthat inversion driving is commonly used in which the polarity of voltageapplied to the liquid crystal element 1409 is inverted per predeterminedperiod. For example, when inversion driving is performed in the periodsF1 to F4, a voltage whose polarity is changed per predetermined period,such as V_(LC) (inversion driving) in FIG. 15B, is applied to the liquidcrystal element 1409 even when the same image is displayed.

When the refresh rate is reduced in order to decrease power consumptionin displaying a still image, each of the periods F1 to F4 is extended.As the period is extended, the voltage (V_(pix)) held in the pixelelectrode 1411 is changed to rise or fall from V_(data) (indicated by anarrow 1502 or an arrow 1503 in FIG. 15B) due to the off-state currentI_(TFT) and/or the current I_(LC) flowing through the liquid crystal. Onthe other hand, the voltage V_(com) of the counter electrode 1412 isfixed. The voltage V_(LC) that is actually applied to the liquid crystal1413 is significantly changed at the boundary between the periods F1 toF4 (denoted by “refresh” in FIG. 15B), which contributes to imagedeterioration when a still image is displayed.

In view of the above, an object of one embodiment of the presentinvention is to suppress deterioration of a displayed image even when arefresh rate is reduced in displaying a still image.

One embodiment of the present invention is a method for driving a liquidcrystal display device. The liquid crystal display device includes apixel transistor electrically connected to a pixel electrode, and acapacitor having one of electrodes electrically connected to the pixelelectrode and the other of the electrodes electrically connected to acapacitor line. The pixel transistor is turned on and a voltage based onan image signal is supplied to the pixel electrode, and then, the pixeltransistor is turned off so that a holding period during which the pixelelectrode holds the voltage based on the image signal starts. A holdingsignal corresponding to change of the voltage based on the image signalin the pixel electrode in the holding period is supplied to thecapacitor line so that a potential of the pixel electrode is constant.

One embodiment of the present invention is a method for driving a liquidcrystal display device. The liquid crystal display device includes apixel transistor electrically connected to a pixel electrode, and acapacitor having one of electrodes electrically connected to the pixelelectrode and the other of the electrodes electrically connected to acapacitor line. The pixel transistor is turned on and a voltage based onan image signal is supplied to the pixel electrode, and then, the pixeltransistor is turned off so that a holding period during which the pixelelectrode holds the voltage based on the image signal starts. When thevoltage based on the image signal in the pixel electrode rises in theholding period, a holding signal for controlling so as to lower thevoltage based on the image signal is supplied to the capacitor line sothat a potential of the pixel electrode is constant.

One embodiment of the present invention is a method for driving a liquidcrystal display device. The liquid crystal display device includes apixel transistor electrically connected to a pixel electrode, and acapacitor having one of electrodes electrically connected to the pixelelectrode and the other of the electrodes electrically connected to acapacitor line. The pixel transistor is turned on and a voltage based onan image signal is supplied to the pixel electrode, and then, the pixeltransistor is turned off so that a holding period during which the pixelelectrode holds the voltage based on the image signal starts. When thevoltage based on the image signal in the pixel electrode falls in theholding period, a holding signal for controlling so as to raise thevoltage based on the image signal is supplied to the capacitor line sothat a potential of the pixel electrode is constant.

In the method for driving a liquid crystal display device according toone embodiment of the present invention, a semiconductor layer of thepixel transistor may be an oxide semiconductor.

In the method for driving a liquid crystal display device according toone embodiment of the present invention, the holding period may be 60seconds or longer.

In the method for driving a liquid crystal display device according toone embodiment of the present invention, the liquid crystal displaydevice may be driven with frame inversion driving, common inversiondriving, source line inversion driving, gate line inversion driving, ordot inversion driving per frame period.

According to one embodiment of the present invention, deterioration of adisplayed image can be suppressed even when the refresh rate is reducedin displaying a still image.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C each illustrate a diagram for explaining a circuitdiagram of one embodiment of the present invention;

FIGS. 2A and 2B are diagrams for explaining a timing chart of oneembodiment of the present invention;

FIG. 3 illustrates a diagram for explaining an example of properties ofa liquid crystal in one embodiment of the present invention;

FIG. 4 illustrates a diagram for explaining a block diagram of oneembodiment of the present invention;

FIG. 5 illustrates a diagram for explaining a circuit diagram of oneembodiment of the present invention;

FIGS. 6A to 6D each illustrate a diagram for explaining a schematicdiagram of one embodiment of the present invention;

FIGS. 7A to 7C each illustrate a diagram for explaining a circuitdiagram of one embodiment of the present invention;

FIGS. 8A and 8B are diagrams for explaining a timing chart of oneembodiment of the present invention;

FIGS. 9A to 9D each illustrate a diagram for explaining a transistor ofone embodiment of the present invention;

FIGS. 10A1, 10A2, and 10B each illustrate a diagram for explaining aliquid crystal display device of one embodiment of the presentinvention;

FIG. 11 illustrates a diagram for explaining a liquid crystal displaydevice of one embodiment of the present invention;

FIGS. 12A to 12D each illustrate a diagram for explaining an electronicdevice of one embodiment of the present invention;

FIGS. 13A to 13D each illustrate a diagram for explaining an electronicdevice of one embodiment of the present invention;

FIGS. 14A to 14C are diagrams for explaining an object; and

FIGS. 15A and 15B are diagrams for explaining an object.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below withreference to the accompanying drawings. Note that the present inventioncan be carried out in many different modes, and it is easily understoodby those skilled in the art that modes and details of the presentinvention can be modified in various ways without departing from thespirit and the scope of the present invention. Therefore, the presentinvention is not construed as being limited to the following descriptionof the embodiments. Note that in structures of the present inventiondescribed below, reference numerals denoting the same portions are usedin common in different drawings.

Note that the size of a component, the thickness of a layer, a region,or distortion of signal waveform illustrated in drawings in embodimentsis exaggerated for simplicity in some cases. Therefore, embodiments ofthe present invention are not limited to such scales.

Note that terms “first”, “second”, “third” to “Nth” (N is a naturalnumber) employed in this specification are used in order to avoidconfusion between components and do not set a limitation on number.

Embodiment 1

For explaining this embodiment, FIG. 1A illustrates a schematic diagramof a display panel in a liquid crystal display device. A display panel100 in FIG. 1A includes a pixel portion 101, a gate line (also referredto as a scan line) 102, a signal line (also referred to as a data line)103, a pixel 104, a common electrode 105, a capacitor line 106, aterminal portion 107, a gate line driver circuit 102D, and a signal linedriver circuit 103D.

Note that FIG. 1A illustrates the structure in which the gate linedriver circuit 102D and the signal line driver circuit 103D are providedover the display panel 100; as in FIG. 14A, the gate line driver circuit102D and the signal line driver circuit 103D are not necessarilyprovided over the display panel 100. When the gate line driver circuit102D and the signal line driver circuit 103D are provided over thedisplay panel 100, the number of terminals in the terminal portion 107can be reduced and the size of the liquid crystal display device can bereduced.

The pixels 104 are arranged (placed) in matrix. Here, the expression“pixels are arranged (placed) in matrix” includes the case where thepixels are arranged in a straight line and the case where the pixels arearranged in a jagged line, in a longitudinal direction or a lateraldirection. Accordingly, in the case of performing full color displaywith three color elements (e.g., RGB), the expression “pixels arearranged (placed) in matrix” also includes the case where pixels arearranged in stripes and the case where dots of the three color elementsare arranged in a delta pattern.

FIG. 1B illustrates the pixel 104 in FIG. 1A. The pixel 104 includes apixel transistor 108, a liquid crystal element 109, and a capacitor 110.A gate of the pixel transistor 108 is connected to the gate line 102. Afirst terminal serving as one of a source and a drain of the pixeltransistor 108 is connected to the signal line 103. A second terminalserving as the other of the source and the drain of the pixel transistor108 is connected to one electrode of the liquid crystal element 109 anda first electrode of the capacitor 110. The other electrode of theliquid crystal element 109 is connected to the common electrode 105. Asecond electrode of the capacitor 110 is connected to the capacitor line106. Note that the pixel transistor 108 is a thin film transistor (TFT)including a thin semiconductor layer.

Note that when it is explicitly described that “A and B are connected,”the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein.

Note that as the pixel transistor 108, a thin film transistor (TFT)including amorphous silicon, polycrystalline silicon, microcrystalline(also referred to as microcrystal or semi-amorphous) silicon, or singlecrystal silicon can be used. A transistor including a compoundsemiconductor or an oxide semiconductor such as ZnO, a-InGaZnO, SiGe, orGaAs; a thin film transistor obtained by thinning such a compoundsemiconductor or oxide semiconductor; or the like can be used.Accordingly, the manufacturing temperature can be lowered and forexample, such a transistor can be formed at room temperature.

Note that one pixel corresponds to one element whose brightness can becontrolled. Therefore, for example, one pixel corresponds to one colorelement and brightness is expressed with one color element. Accordingly,in the case of a color display device having color elements of R (Red),G (Green), and B (Blue), a minimum unit of an image is composed of threepixels of an R pixel, a G pixel, and a B pixel. Note that a color thatis different from R, G, and B may be used for a color element. Forexample, three pixels of yellow, cyan, and magenta may be used.

Note that a thin film transistor is an element having at least threeterminals of gate, drain, and source. The thin film transistor includesa channel region between a drain region and a source region, and acurrent can flow through the drain region, the channel region, and thesource region. Here, since the source and the drain of the transistormay change depending on the structure, the operating condition, and thelike of the transistor, it is difficult to define which is a source or adrain. Therefore, in this document (the specification, the claims, thedrawings, and the like), a region functioning as a source or a drain isnot called a source or a drain in some cases. In such a case, forexample, one of the source and the drain is referred to as a firstterminal, a first electrode, or a source region and the other of thesource and the drain is referred to as a second terminal, a secondelectrode, or a drain region in some cases.

FIG. 1C illustrates the pixel 104 in a manner different from FIG. 1B inorder to focus on each of the wirings and the elements. The referencenumerals of the wirings and the elements are the same as those in FIG.1B. Note that for description of the liquid crystal element 109, FIG. 1Cillustrates a pixel electrode 111 as the electrode on the pixeltransistor 108 side, a counter electrode 112 as the electrode on thecommon electrode 105 side, and a liquid crystal 113 placed between thepixel electrode 111 and the counter electrode 112. FIG. 1C differs fromFIG. 14C in that instead of supplying a fixed voltage to the capacitorline 106 connected to the second electrode of the capacitor 110, asignal whose voltage is changed per predetermined period is supplied tothe second electrode of the capacitor 110.

FIG. 2A illustrates the same diagram as FIG. 1C, and focuses on apotential of each wiring, a voltage between electrodes, and a currentflowing through each element as in FIG. 15A. An image signal supplied tothe signal line 103 is a voltage V_(data). A signal supplied to thecapacitor line 106 (a holding signal) is a voltage V_(cap). A voltageheld in the pixel electrode 111 is V_(pix). A voltage of the counterelectrode 112 is V_(com). A voltage applied to the liquid crystal 113 isV_(LC). FIG. 2A also illustrates an off-state current I_(TFT) of thepixel transistor and a current I_(LC) flowing through the liquid crystal113.

Specifically, there is a period during which the pixel transistor isturned on and the voltage V_(data) based on an image signal is suppliedto the pixel electrode 111 in order to write an image signal into apixel (the period is hereinafter referred to as a writing period), and avoltage held in the pixel electrode 111 is V_(pix). The voltage V_(pix)is held by turning off the pixel transistor. Note that in a period forholding V_(pix) (hereinafter referred to as a holding period), V_(pix)is changed to rise or fall because of the off-state current I_(TFT)and/or the current I_(LC); thus, it is necessary to regularly performrefresh operation. Note that a writing period and a holding period canbe collectively referred to as one frame period.

Note that in this specification, a writing period is extremely shorterthan a holding period. For that reason, in some cases, a writing periodis not shown in a timing chart and a holding period is described as oneframe period.

When a thin film transistor in which an oxide semiconductor is used fora semiconductor layer is used as the pixel transistor, the off-statecurrent I_(TFT) can be extremely reduced. Thus, it is possible to obtaina structure in which only the current I_(LC) flowing through the liquidcrystal 113 is largely contributed to change in voltage V_(pix). As aresult, the holding period can be drastically extended to 60 seconds ormore, and the refresh rate can be significantly reduced.

Note that voltage often refers to a potential difference between a givenpotential and a reference potential (e.g., a ground potential).Accordingly, voltage, potential, and potential difference can bereferred to as potential, voltage, and voltage difference, respectively.

Like FIG. 15B, FIG. 2B illustrates a general timing chart showing thepotential of each wiring and the voltage between electrodes illustratedin FIG. 2A. In the timing chart in FIG. 2B, a period is divided intoperiods F1 to F4. The following description is made on the assumptionthat the same image, that is, a still image is displayed in the periodsF1 to F4. That is, in the periods F1 to F4, the voltage V_(LC) appliedto the liquid crystal 113 is a constant voltage V_(data), (indicated byan arrow 121 in FIG. 2B). It is known that the liquid crystal element109 deteriorates by application of voltage in one direction to theliquid crystal 113, and that inversion driving is commonly used in whichthe polarity of voltage applied to the liquid crystal element isinverted per predetermined period. For example, frame inversion drivingis realized when each of the periods F1 to F4 is regarded as one frameperiod and inversion driving is performed per frame period; a voltagewhose polarity is changed per predetermined period, such as V_(LC)(inversion driving) in FIG. 2B, is applied to the liquid crystal elementeven when the same image is displayed.

When the refresh rate is reduced in order to decrease power consumptionin displaying a still image, each of the periods F1 to F4 is extended.As the period is extended, the voltage V_(pix) held in the pixelelectrode 111 rises or falls because of the off-state current I_(TFT)and/or the current I_(LC) flowing through the liquid crystal, asdescribed in FIG. 15B.

In the structure in this embodiment, image deterioration in displaying astill image is reduced in such a manner that the holding signal V_(cap)compensates a voltage corresponding to the amount of rise or fall fromV_(data) of the voltage (V_(pix)) held in the pixel electrode 111 due tothe off-state current I_(TFT) and/or the current I_(LC) flowing throughthe liquid crystal. Specifically, in the periods F1 to F4 each of whichis one frame period, the voltage of the holding signal V_(cap) is raisedor lowered by the amount of change in voltage V_(pix). (indicated by anarrow 122 or an arrow 123 in FIG. 2B). In other words, the voltage ofthe holding signal V_(cap) is lowered when the voltage V_(pix) ischanged to rise, whereas the voltage of the holding signal V_(cap) israised when the voltage V_(pix) is changed to fall. The voltage V_(LC)applied between the voltage V_(pix) and the voltage V_(com) of thecounter electrode 112 which is a fixed voltage is not much changed atthe boundary between the periods F1 to F4 (denoted by “refresh” in FIG.2B), and image deterioration in displaying a still image can be reduced.Note that V_(LC) (inversion driving) is a voltage that is inverted ineach of the periods F1 to F4, so that V_(cap) is a signal thatalternately repeats rise and fall. Note that the reduction in change ofV_(pix), that is, V_(LC) by control of the voltage of the holding signalV_(cap) as described above can be referred to as “making the voltageV_(pix), that is, the voltage V_(LC) constant”; it is to be noted that“constant” in this case includes minute change in voltage which hardlyaffects actual display.

Note that the amount indicated by the arrow 122 or the arrow 123 thatcorresponds to the amount of change in voltage is changed in accordancewith an image signal. In particular, when an image signal is hardlysupplied to the pixel electrode, the voltage rarely changes. FIG. 3illustrates the relation between a transmittance and voltage applied tothe liquid crystal 113. As seen from FIG. 3, there is no problem in thecase where an image signal is hardly supplied to the pixel electrode,that is, in the case where an applied voltage is low, where thetransmittance corresponding to the applied voltage is hardly changedwhen the applied voltage is changed a little.

FIG. 4 is a block diagram of a liquid crystal display device including acircuit that outputs the holding signal V_(cap). The liquid crystaldisplay device in FIG. 4 includes a display panel portion 301 and aperipheral circuit portion 302. The display panel portion 301 has astructure similar to that of the display panel 100 in FIG. 1A;therefore, the description is not repeated. The peripheral circuitportion 302 includes a circuit 303 for switching between a moving imageand a still image (hereinafter referred to as an image switching circuit303), a display control circuit 304, and a holding signal generationcircuit 305. Note that the display panel portion 301 and the peripheralcircuit portion 302 are preferably formed over different substrates;they may be formed over the same substrate.

The image switching circuit 303 judges whether image signals suppliedfrom the outside are for a moving image or a still image and switches animage between a moving image and a still image. The image switchingcircuit 303 may automatically judge whether image signals supplied fromthe outside are for a moving image or a still image by comparing theimage signals for subsequent frame periods, or may switch an imagebetween a moving image and a still image in accordance with a signalfrom the outside.

The display control circuit 304 supplies a signal for displaying amoving image, for example, an image signal, a clock signal, and the liketo the display panel portion 301 when the image switching circuit 303judges that the image signals are for a moving image. On the other hand,when the image switching circuit 303 judges that the image signals arefor a still image, the display control circuit 304 supplies a signal fordisplaying a still image, for example, an image signal, a clock signal,and the like to the display panel portion 301 at predetermined timingwhile reducing the refresh rate.

The holding signal generation circuit 305 generates the holding signalV_(cap) supplied to the capacitor line 106 when the image switchingcircuit 303 judges that the image signals are for a still image. Whenthe image switching circuit 303 judges that the image signals are for amoving image, the holding signal generation circuit 305 supplies a givenconstant voltage, for example, a signal same as the common voltageV_(com) to the display panel portion 301.

Note that a high power supply potential VDD refers to a potential thatis higher than a reference potential, and a low power supply potentialVSS refers to a potential that is lower than or equal to the referencepotential. Both the high power supply potential and the low power supplypotential are preferably potentials with which a thin film transistorcan operate. Note that the high power supply potential VDD and the lowpower supply potential VSS are collectively referred to as a powersupply voltage in some cases.

An example of the structure of the holding signal generation circuit 305is described with reference to FIG. 5. As an example, the holding signalgeneration circuit 305 illustrated in FIG. 5 includes a first currentsource circuit 501, a first switch 502, a second switch 503, a secondcurrent source circuit 504, and a third switch 505. The holding signalgeneration circuit 305 in FIG. 5 controls rise or fall in voltage of thecapacitor line 106 by the first current source circuit 501 and thesecond current source circuit 504 in such a manner that on/off of thefirst switch 502 and the second switch 503 is alternately switched bycontrol of a switching terminal 507 in a period during which a stillimage is displayed. Note that when the voltage of the capacitor line 106is a given constant voltage, the third switch 505 is turned on so thatthe capacitor line 106 is connected to a terminal 506 to which thecommon voltage V_(com) is supplied.

Note that it is preferable that the first switch 502, the second switch503, and the third switch 505 be transistors, and the first switch 502and the second switch 503 be transistors with opposite polarities.

As described above, the structure shown in this embodiment can suppressdeterioration of a displayed image even when the refresh rate is reducedin displaying a still image.

This embodiment can be implemented in appropriate combination with anyof the components described in the other embodiments.

Embodiment 2

In this embodiment, a structure different from the structure describedin Embodiment 1 will be described.

Embodiment 1 describes the structure for frame inversion drivingillustrated in FIG. 6A; this embodiment explains source line inversiondriving in which inversion driving with the polarity inverted per signalline is performed as illustrated in FIG. 6B, gate line inversion drivingin which inversion driving with the polarity inverted per gate line isperformed as illustrated in FIG. 6C, and dot inversion driving in whichinversion driving with inverted polarity is performed between adjacentpixels as illustrated in FIG. 6D, by using a circuit configuration of apixel, and the like. Note that the part of the same description as thatin Embodiment 1 is not repeated. Operation for common inversion drivingis the same as that for frame inversion driving; therefore, descriptionof common inversion driving is omitted. Note that FIGS. 6A to 6Dillustrate examples in which image signals with polarities invertedbetween an Nth frame (N is a natural number) and a (N+1)th frame aresupplied (the polarity is denoted by a plus sign or a minus sign inFIGS. 6A to 6D); alternatively, another driving method may be employed.

The methods for inversion driving in FIGS. 6B to 6D are different fromthe method in FIG. 6A, which is the inversion driving in Embodiment 1,in that image signals of different polarities are supplied in one frameperiod; accordingly, a voltage supplied to a capacitor line is changeddepending on the polarity of an image signal.

Specific description is made using simple circuit configurations. FIG.7A illustrates a circuit configuration of a pixel for source lineinversion driving, corresponding to FIG. 6B. FIG. 7A illustrates a gateline 102, a signal line 103, a pixel 104, a common electrode 105, afirst capacitor line 106A, and a second capacitor line 106B. The firstcapacitor line 106A is connected to pixels to which image signals of onepolarity are supplied, and the second capacitor line 106B is connectedto other pixels to which image signals of a different polarity aresupplied as illustrated in FIG. 6B. FIG. 7B illustrates a circuitconfiguration of a pixel for gate line inversion driving, correspondingto FIG. 6C. FIG. 7B illustrates a gate line 102, a signal line 103, apixel 104, a common electrode 105, a first capacitor line 106A, and asecond capacitor line 106B. The first capacitor line 106A is connectedto pixels to which image signals of one polarity are supplied, and thesecond capacitor line 106B is connected to other pixels to which imagesignals of a different polarity are supplied as illustrated in FIG. 6C.FIG. 7C illustrates a circuit configuration of a pixel for dot inversiondriving, corresponding to FIG. 6D. FIG. 7C illustrates a gate line 102,a signal line 103, a pixel 104, a common electrode 105, a firstcapacitor line 106A, and a second capacitor line 106B. The firstcapacitor line 106A is connected to pixels to which image signals of onepolarity are supplied, and the second capacitor line 106B is connectedto other pixels to which image signals of the different polarity aresupplied as illustrated in FIG. 6D. The above-described first capacitorline 106A and second capacitor line 106B in FIGS. 7A to 7C are suppliedwith a first holding signal V_(cap1) and a second holding signalV_(cap2) which are different holding signals.

FIG. 8A illustrates the same diagram as FIG. 2A and focuses on apotential of each wiring, a voltage between electrodes, and a currentflowing through each element. The difference from FIG. 2A is that FIG.8A illustrates the first holding signal V_(cap1) and the second holdingsignal V_(cap2) described with reference to FIGS. 7A to 7C.

Like FIG. 2B, FIG. 8B illustrates a general timing chart showing thepotential of each wiring and the voltage between electrodes illustratedin FIG. 8A. Image signals of different polarities are supplied to theliquid crystal element 109, and the liquid crystal 113 is supplied witha first voltage V_(LC) and a second voltage V_(LC) that are inverted perframe period. Then, the first holding signal V_(cap1) or the secondholding signal V_(cap2) that compensates the amount of rise or fall inthe voltage (V_(pix)) held in the pixel electrode 111 from V_(data) dueto the off-state current I_(TFT) and/or the current I_(LC) flowingthrough the liquid crystal is supplied. The first V_(pix) and the secondV_(pix) in a pixel to which image signals of different polarities aresupplied in the periods F1 to F4 each of which is one frame period arenot much changed at the boundary between the periods F1 to F4 (denotedby “refresh” in FIG. 8B); thus, image deterioration in displaying astill image can be reduced.

As described above, the structure shown in this embodiment can suppressdeterioration of a displayed image even when the refresh rate is reducedin displaying a still image.

This embodiment can be implemented in appropriate combination with anyof the components described in the other embodiments.

Embodiment 3

In this embodiment, an example of a transistor that can be applied to aliquid crystal display device disclosed in this specification will bedescribed.

FIGS. 9A to 9D each illustrate an example of a cross-sectional structureof a transistor.

A transistor 410 illustrated in FIG. 9A is a kind of bottom-gatestructure thin film transistor and is also called an inverted staggeredthin film transistor.

The transistor 410 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, a gate insulating layer 402, anoxide semiconductor layer 403, a source electrode layer 405 a, and adrain electrode layer 405 b. An insulating layer 407 is provided tocover the transistor 410 and be stacked over the oxide semiconductorlayer 403. A protective insulating layer 409 is provided over theinsulating layer 407.

A transistor 420 illustrated in FIG. 9B has a kind of bottom-gatestructure called a channel-protective type (channel-stop type) and isalso referred to as an inverted staggered thin film transistor.

The transistor 420 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, a gate insulating layer 402, anoxide semiconductor layer 403, an insulating layer 427 that is providedover a channel formation region in the oxide semiconductor layer 403 andfunctions as a channel protective layer, a source electrode layer 405 a,and a drain electrode layer 405 b. A protective insulating layer 409 isprovided to cover the transistor 420.

A transistor 430 illustrated in FIG. 9C is a bottom-gate type thin filmtransistor and includes, over a substrate 400 which is a substratehaving an insulating surface, a gate electrode layer 401, a gateinsulating layer 402, a source electrode layer 405 a, a drain electrodelayer 405 b, and an oxide semiconductor layer 403. An insulating layer407 is provided to cover the transistor 430 and be in contact with theoxide semiconductor layer 403. A protective insulating layer 409 isprovided over the insulating layer 407.

In the transistor 430, the gate insulating layer 402 is provided incontact with the substrate 400 and the gate electrode layer 401. Thesource electrode layer 405 a and the drain electrode layer 405 b areprovided in contact with the gate insulating layer 402. The oxidesemiconductor layer 403 is provided over the gate insulating layer 402,the source electrode layer 405 a, and the drain electrode layer 405 b.

A transistor 440 illustrated in FIG. 9D is a kind of top-gate structurethin film transistor. The transistor 440 includes, over a substrate 400having an insulating surface, an insulating layer 447, an oxidesemiconductor layer 403, a source electrode layer 405 a and a drainelectrode layer 405 b, a gate insulating layer 402, and a gate electrodelayer 401. A wiring layer 446 a and a wiring layer 446 b are provided incontact with the source electrode layer 405 a and the drain electrodelayer 405 b, respectively, to be electrically connected to the sourceelectrode layer 405 a and the drain electrode layer 405 b, respectively.

In this embodiment, the oxide semiconductor layer 403 is used as asemiconductor layer.

As the oxide semiconductor layer 403, any of the following oxidesemiconductor layers can be used: a quaternary metal oxide film such asan In—Sn—Ga—Zn—O film; a ternary metal oxide film such as an In—Ga—Zn—Ofilm, an In—Sn—Zn—O film, an In—Al—Zn—O film, a Sn—Ga—Zn—O film, anAl—Ga—Zn—O film, or a Sn—Al—Zn—O film; a binary metal oxide film such asan In—Zn—O film, a Sn—Zn—O film, an Al—Zn—O film, a Zn—Mg—O film, aSn—Mg—O film, or an In—Mg—O film; an In—O film, a Sn—O film, or a Zn—Ofilm. Further, the above-described oxide semiconductor layer may containSiO₂.

As the oxide semiconductor layer 403, a thin film expressed byInMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one or more metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, or Ga and Co. An oxide semiconductor film whosecomposition formula is represented by InMO₃(ZnO)_(m) (m>0) where atleast Ga is contained as M is referred to as an In—Ga—Zn—O oxidesemiconductor, and a thin film thereof is also referred to as anIn—Ga—Zn—O film.

Note that in the structure in this embodiment, the oxide semiconductoris an intrinsic (i-type) or substantially intrinsic semiconductorobtained by removal of hydrogen, which is an n-type impurity, from theoxide semiconductor for high purification so that the oxidesemiconductor contains an impurity other than the main component aslittle as possible. In other words, the oxide semiconductor in thisembodiment is a highly purified i-type (intrinsic) semiconductor or asubstantially intrinsic semiconductor obtained by removing impuritiessuch as hydrogen and water as much as possible, not by adding animpurity element. Therefore, the oxide semiconductor layer included inthe thin film transistor is a highly purified and electrically i-type(intrinsic) oxide semiconductor layer.

The number of carriers in the highly purified oxide semiconductor isvery small (close to zero), and the carrier concentration is less than1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, further preferably lessthan 1×10¹¹/cm³.

The number of carriers in the oxide semiconductor is so small that theoff-state current of the transistor can be reduced. Specifically, theoff-state current of the thin film transistor including the oxidesemiconductor layer (per channel width of 1 μm) can be reduced to 10aA/μm (1×10⁻¹⁷ A/μm) or lower, further reduced to 1 aA/μm (1×10⁻¹⁸ A/μm)or lower, and still further reduced to 10 zA/μm (1×10⁻²⁰ A/μm). In otherwords, in circuit design, the oxide semiconductor can be regarded as aninsulator when the transistor is off. Moreover, when the thin filmtransistor is on, the current supply capability of the oxidesemiconductor layer is expected to be higher than that of asemiconductor layer formed of amorphous silicon.

In each of the transistors 410, 420, 430, and 440 including the oxidesemiconductor layer 403, the current in an off state (the off-statecurrent) can be small. Thus, the retention time for an electric signalsuch as image data can be extended, and an interval between writings canbe extended. As a result, the frequency of refresh can be reduced, sothat power consumption can be further reduced.

Furthermore, the transistors 410, 420, 430, and 440 including the oxidesemiconductor layer 403 can have relatively high field-effect mobilityas the ones formed using an amorphous semiconductor, thus, thetransistors can operate at high speed. As a result, high functionalityand high-speed response of a display device can be realized.

Although there is no particular limitation on a substrate that can beused as the substrate 400 having an insulating surface, the substrateneeds to have heat resistance at least high enough to withstand heattreatment to be performed later. A glass substrate made of bariumborosilicate glass, aluminoborosilicate glass, or the like can be used.

In the case where the temperature of heat treatment to be performedlater is high, a glass substrate whose strain point is greater than orequal to 730° C. is preferably used. For a glass substrate, a glassmaterial such as aluminosilicate glass, aluminoborosilicate glass, orbarium borosilicate glass is used, for example. Note that a glasssubstrate containing a larger amount of barium oxide (BaO) than boronoxide (B₂O₃), which is practical heat-resistant glass, may be used.

Note that a substrate formed of an insulator, such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate, may be usedinstead of the glass substrate. Alternatively, crystallized glass or thelike may be used. A plastic substrate or the like can be used asappropriate.

In the bottom-gate structure transistors 410, 420, and 430, aninsulating film serving as a base film may be provided between thesubstrate and the gate electrode layer. The base film has a function ofpreventing diffusion of an impurity element from the substrate, and canbe formed with a single-layer structure or a layered structure includinga silicon nitride film, a silicon oxide film, a silicon nitride oxidefilm, and/or a silicon oxynitride film.

The gate electrode layer 401 can be formed with a single-layer structureor a layered structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium or an alloy material containing any of these materials as itsmain component.

As a two-layer structure of the gate electrode layer 401, any of thefollowing layered structures is preferably employed, for example: atwo-layer structure in which a molybdenum layer is stacked over analuminum layer, a two-layer structure in which a molybdenum layer isstacked over a copper layer, a two-layer structure in which a titaniumnitride layer or a tantalum nitride layer is stacked over a copperlayer, or a two-layer structure in which a titanium nitride layer and amolybdenum layer are stacked. As a three-layer structure of the gateelectrode layer 401, it is preferable to employ a stack of a tungstenlayer or a tungsten nitride layer, a layer of an alloy of aluminum andsilicon or an alloy of aluminum and titanium, and a titanium nitridelayer or a titanium layer. Note that the gate electrode layer can beformed using a light-transmitting conductive film. An example of amaterial for the light-transmitting conductive film is alight-transmitting conductive oxide.

The gate insulating layer 402 can be formed with a single-layerstructure or a layered structure using any of a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, an aluminum oxide layer, an aluminum nitride layer, analuminum oxynitride layer, an aluminum nitride oxide layer, and ahafnium oxide layer by a plasma CVD method, a sputtering method, or thelike.

The gate insulating layer 402 can have a structure in which a siliconnitride layer and a silicon oxide layer are stacked from the gateelectrode layer side. For example, a 100-nm-thick gate insulating layeris formed in such a manner that a silicon nitride layer (SiN_(y) (y>0))having a thickness of 50 nm to 200 nm is formed as a first gateinsulating layer by a sputtering method and then a silicon oxide layer(SiO_(x) (x>0)) having a thickness of 5 nm to 300 nm is stacked as asecond gate insulating layer over the first gate insulating layer. Thethickness of the gate insulating layer 402 may be set as appropriatedepending on characteristics needed for a thin film transistor, and maybe approximately 350 nm to 400 nm.

For a conductive film used for the source electrode layer 405 a and thedrain electrode layer 405 b, an element selected from Al, Cr, Cu, Ta,Ti, Mo, and W, an alloy containing any of these elements, or an alloyfilm containing a combination of any of these elements can be used, forexample. A structure may be employed in which a high-melting-point metallayer of Cr, Ta, Ti, Mo, W, or the like is stacked on one or both of atop surface and a bottom surface of a metal layer of Al, Cu, or thelike. By using an aluminum material to which an element preventinggeneration of hillocks and whiskers in an aluminum film, such as Si, Ti,Ta, W, Mo, Cr, Nd, Sc, or Y, is added, heat resistance can be increased.

A conductive film serving as the wiring layers 446 a and 446 b connectedto the source electrode layer 405 a and the drain electrode layer 405 bcan be formed using a material similar to that of the source and drainelectrode layers 405 a and 405 b.

The source electrode layer 405 a and the drain electrode layer 405 b mayhave a single-layer structure or a layered structure of two or morelayers. For example, the source electrode layer 405 a and the drainelectrode layer 405 b can have a single-layer structure of an aluminumfilm containing silicon, a two-layer structure in which a titanium filmis stacked over an aluminum film, or a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder.

The conductive film to be the source electrode layer 405 a and the drainelectrode layer 405 b (including a wiring layer formed using the samelayer as the source and drain electrode layers) may be formed using aconductive metal oxide. As the conductive metal oxide, indium oxide(In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), an alloy of indium oxideand tin oxide (In₂O₃—SnO₂, referred to as ITO), an alloy of indium oxideand zinc oxide (In₂O₃—ZnO), or any of the metal oxide materialscontaining silicon or silicon oxide can be used.

As the insulating layers 407, 427, and 447 and the protective insulatinglayer 409, an inorganic insulating film such as an oxide insulatinglayer or a nitride insulating layer is preferably used.

As the insulating layers 407, 427, and 447, an inorganic insulating filmsuch as a silicon oxide film, a silicon oxynitride film, an aluminumoxide film, or an aluminum oxynitride film can be typically used.

As the protective insulating layer 409, an inorganic insulating filmsuch as a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, or an aluminum nitride oxide film can be used.

A planarization insulating film may be formed over the protectiveinsulating layer 409 in order to reduce surface roughness due to thetransistor. The planarization insulating film can be formed using aheat-resistant organic material such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy. Other than such organicmaterials, it is possible to use a low-dielectric constant material (alow-k material), a siloxane-based resin, PSG (phosphosilicate glass),BPSG (borophosphosilicate glass), or the like. Note that theplanarization insulating film may be formed by stacking a plurality ofinsulating films formed from these materials.

By using the transistor including the oxide semiconductor layer in thisembodiment, it is possible to provide a highly functional liquid crystaldisplay device with lower power consumption.

This embodiment can be implemented in appropriate combination with anyof the components described in the other embodiments.

Embodiment 4

When thin film transistors are manufactured and used for a pixel portionand a driver circuit, a liquid crystal display device having a displayfunction can be manufactured. Further, part of or the entire drivercircuit can be formed over a substrate where a pixel portion is formed,using a thin film transistor; thus, a system-on-panel can be obtained.

Note that the liquid crystal display device includes any of thefollowing modules in its category: a module provided with a connector,for example, a flexible printed circuit (FPC), a tape automated bonding(TAB) tape, or a tape carrier package (TCP); a module provided with aprinted wiring board at the end of a TAB tape or a TCP; and a modulewhere an integrated circuit (IC) is directly mounted on a displayelement by a chip-on-glass (COG) method.

The appearance and a cross section of a liquid crystal display devicewill be described with reference to FIGS. 10A1, 10A2, and 10B. FIGS.10A1 and 10A2 are plan views of panels in which thin film transistors4010 and 4011 and a liquid crystal element 4013 are sealed between afirst substrate 4001 and a second substrate 4006 with a sealant 4005.FIG. 10B is a cross-sectional view along M-N in FIGS. 10A1 and 10A2.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda gate line driver circuit 4004 that are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the gate line driver circuit 4004. Therefore, the pixelportion 4002 and the gate line driver circuit 4004 are sealed togetherwith a liquid crystal layer 4008, by the first substrate 4001, thesealant 4005, and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofa driver circuit that is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 10A1illustrates an example where the signal line driver circuit 4003 ismounted by a COG method. FIG. 10A2 illustrates an example where thesignal line driver circuit 4003 is mounted by a TAB method.

The pixel portion 4002 and the gate line driver circuit 4004 providedover the first substrate 4001 include a plurality of thin filmtransistors. FIG. 10B illustrates the thin film transistor 4010 includedin the pixel portion 4002 and the thin film transistor 4011 included inthe gate line driver circuit 4004. Insulating layers 4041 a, 4041 b,4042 a, 4042 b, 4020, and 4021 are provided over the thin filmtransistors 4010 and 4011.

A highly reliable thin film transistor including an oxide semiconductorlayer can be used as the thin film transistors 4010 and 4011. In thisembodiment, the thin film transistors 4010 and 4011 are n-channel thinfilm transistors.

A conductive layer 4040 is provided over part of the insulating layer4021, which overlaps with a channel formation region of an oxidesemiconductor layer in the thin film transistor 4011 for the drivercircuit. The conductive layer 4040 is provided at the positionoverlapping with the channel formation region of the oxide semiconductorlayer, so that the amount of change in threshold voltage of the thinfilm transistor 4011 before and after the BT (bias-temperature) test canbe reduced. The potential of the conductive layer 4040 may be the sameor different from that of a gate electrode layer of the thin filmtransistor 4011. The conductive layer 4040 can also function as a secondgate electrode layer. The potential of the conductive layer 4040 may beGND or 0 V, or the conductive layer 4040 may be in a floating state.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is provided forthe second substrate 4006. A portion where the pixel electrode layer4030, the counter electrode layer 4031, and the liquid crystal layer4008 overlap with one another corresponds to the liquid crystal element4013. Note that the pixel electrode layer 4030 and the counter electrodelayer 4031 are provided with an insulating layer 4032 and an insulatinglayer 4033 functioning as alignment films, respectively, and the liquidcrystal layer 4008 is sandwiched between the pixel electrode layer 4030and the counter electrode layer 4031 with the insulating layers 4032 and4033 therebetween.

Note that a light-transmitting substrate can be used as the firstsubstrate 4001 and the second substrate 4006; glass, ceramics, orplastics can be used. As plastics, a fiberglass-reinforced plastics(FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or anacrylic resin film can be used.

A spacer 4035 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the distance (a cellgap) between the pixel electrode layer 4030 and the counter electrodelayer 4031. Note that a spherical spacer may be used. The counterelectrode layer 4031 is electrically connected to a common potentialline formed over the substrate where the thin film transistor 4010 isformed. With use of the common connection portion, the counter electrodelayer 4031 and the common potential line can be electrically connectedto each other by conductive particles arranged between a pair ofsubstrates. Note that the conductive particles can be included in thesealant 4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase is only generated within anarrow range of temperature, a liquid crystal composition containing achiral agent at 5 wt % or more so as to improve the temperature range isused for the liquid crystal layer 4008. The liquid crystal compositionthat includes a liquid crystal exhibiting a blue phase and a chiralagent has a short response time of 1 msec or less, has optical isotropy,which makes the alignment process unneeded, and has a small viewingangle dependence.

Note that this embodiment can also be applied to a transflective liquidcrystal display device in addition to a transmissive liquid crystaldisplay device.

This embodiment shows the example of the liquid crystal display devicein which a polarizing plate is provided on the outer side of thesubstrate (on the viewer side) and a coloring layer and an electrodelayer used for a display element are provided in this order on the innerside of the substrate; alternatively, a polarizing plate may be providedon the inner side of the substrate. The layered structure of thepolarizing plate and the coloring layer is not limited to that in thisembodiment and may be set as appropriate depending on materials of thepolarizing plate and the coloring layer or conditions of themanufacturing process. Further, a light-blocking film serving as a blackmatrix may be provided in a portion other than a display portion.

The insulating layer 4041 a that serves as a channel protective layerand the insulating layer 4041 b that covers an outer edge portion(including a side surface) of the stack of the oxide semiconductorlayers are formed in the thin film transistor 4011. In a similar manner,the insulating layer 4042 a that serves as a channel protective layerand the insulating layer 4042 b that covers an outer edge portion(including a side surface) of the stack of the oxide semiconductorlayers are formed in the thin film transistor 4010.

The insulating layers 4041 b and 4042 b that are oxide insulating layerscovering the outer edge portion (including the side surface) of thestack of the oxide semiconductor layers can increase the distancebetween the gate electrode layer and a wiring layer (e.g., a sourcewiring layer or a capacitor wiring layer) formed over or around the gateelectrode layer, so that the parasitic capacitance can be reduced. Inorder to reduce the surface roughness of the thin film transistors, thethin film transistors are covered with the insulating layer 4021 servingas a planarizing insulating film. Here, as the insulating layers 4041 a,4041 b, 4042 a, and 4042 b, a silicon oxide film is formed by asputtering method, for example.

Moreover, the insulating layer 4020 is formed over the insulating layers4041 a, 4041 b, 4042 a, and 4042 b. As the insulating layer 4020, asilicon nitride film is formed by an RF sputtering method, for example.

The insulating layer 4021 is formed as the planarizing insulating film.As the insulating layer 4021, an organic material having heatresistance, such as polyimide, acrylic, benzocyclobutene, polyamide, orepoxy can be used. Other than such organic materials, it is alsopossible to use a low-dielectric constant material (a low-k material), asiloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. Note that the insulating layer4021 may be formed by stacking a plurality of insulating films formed ofthese materials.

In this embodiment, a plurality of thin film transistors in the pixelportion may be surrounded together by a nitride insulating film. It ispossible to use a nitride insulating film as the insulating layer 4020and the gate insulating layer and to provide a region where theinsulating layer 4020 is in contact with the gate insulating layer so asto surround at least the periphery of the pixel portion in the activematrix substrate. In this manufacturing process, entry of moisture fromthe outside can be prevented. Further, even after the device iscompleted as a liquid crystal display device, entry of moisture from theoutside can be prevented in the long term, and the long-term reliabilityof the device can be improved.

Note that a siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include an organic group (e.g.,an alkyl group or an aryl group) or a fluoro group as a substituent. Theorganic group may include a fluoro group.

There is no particular limitation on the formation method of theinsulating layer 4021, and any of the following methods and tools can beemployed, for example, depending on the material: a sputtering method,an SOG method, a spin coating method, a dipping method, a spray coatingmethod, a droplet discharge method (e.g., an ink-jet method, screenprinting, and offset printing), a doctor knife, a roll coater, a curtaincoater, and a knife coater. The baking step of the insulating layer 4021also serves as annealing of the semiconductor layer, so that a liquidcrystal display device can be efficiently manufactured.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (referred to as ITO), indiumzinc oxide, or indium tin oxide to which silicon oxide is added.

Alternatively, the pixel electrode layer 4030 and the counter electrodelayer 4031 can be formed using a conductive composition including aconductive high molecule (also referred to as a conductive polymer). Thepixel electrode formed using the conductive composition preferably has asheet resistance of less than or equal to 10000 ohms per square and atransmittance of greater than or equal to 70% at a wavelength of 550 nm.Further, the resistivity of the conductive high molecule included in theconductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive high molecule can be used. Examples are polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, and a copolymer of two or more of thesematerials.

A variety of signals and potentials are supplied from an FPC 4018 to thesignal line driver circuit 4003 which is formed separately, the gateline driver circuit 4004, or the pixel portion 4002.

A connection terminal electrode 4015 is formed from the same conductivefilm as the pixel electrode layer 4030 included in the liquid crystalelement 4013, and a terminal electrode 4016 is formed from the sameconductive film as source and drain electrode layers of the thin filmtransistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 via an anisotropic conductive film4019.

Note that FIGS. 10A1 and 10A2 illustrate the example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001; however, the this embodiment is not limited to thisstructure. The gate line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe gate line driver circuit may be separately formed and then mounted.

FIG. 11 illustrates an example of a structure of a liquid crystaldisplay device.

FIG. 11 illustrates an example of a liquid crystal display device. A TFTsubstrate 2600 and a counter substrate 2601 are fixed to each other witha sealant 2602. A pixel portion 2603 including a TFT and the like, adisplay element 2604 including a liquid crystal layer, and a coloringlayer 2605 are provided between the substrates so that a display regionis formed. The coloring layer 2605 is necessary to perform colordisplay. In the RGB system, coloring layers corresponding to colors ofred, green, and blue are provided for pixels. A polarizing plate 2606 isprovided on the outer side of the counter substrate 2601. A polarizingplate 2607 and a diffusion plate 2613 are provided on the outer side ofthe TFT substrate 2600. A light source includes a cold cathode tube 2610and a reflective plate 2611. A circuit board 2612 is connected to awiring circuit portion 2608 of the TFT substrate 2600 by a flexiblewiring board 2609 and includes an external circuit such as a controlcircuit or a power source circuit. The polarizing plate and the liquidcrystal layer may be stacked with a retardation plate therebetween.

For a method for driving the liquid crystal display device, a TN(twisted nematic) mode, an IPS (in-plane-switching) mode, an FFS (fringefield switching) mode, an MVA (multi-domain vertical alignment) mode, aPVA (patterned vertical alignment) mode, an ASM (axially symmetricaligned micro-cell) mode, an OCB (optically compensated birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(antiferroelectric liquid crystal) mode, or the like can be used.

Through the above-described process, it is possible to manufacture aliquid crystal display device in which deterioration of a displayedimage can be reduced in displaying a still image.

This embodiment can be implemented in appropriate combination with anyof the components described in the other embodiments.

Embodiment 5

In this embodiment, an example of an electronic device including theliquid crystal display device described in any of the above-describedembodiments will be described.

FIG. 12A illustrates a portable game machine that can include a housing9630, a display portion 9631, a speaker 9633, operation keys 9635, aconnection terminal 9636, a recording medium reading portion 9672, andthe like. The portable game machine in FIG. 12A can have a function ofreading a program or data stored in the recording medium to display iton the display portion, a function of sharing information with anotherportable game machine by wireless communication, and the like. Note thatthe functions of the portable game machine in FIG. 12A are not limitedto those described above, and the portable game machine can have variousfunctions.

FIG. 12B illustrates a digital camera that can include a housing 9630, adisplay portion 9631, a speaker 9633, operation keys 9635, a connectionterminal 9636, a shutter button 9676, an image receiving portion 9677,and the like. The digital camera in FIG. 12B can have a function ofphotographing a still image and/or a moving image, a function ofautomatically or manually correcting the photographed image, a functionof obtaining various kinds of information from an antenna, a function ofsaving the photographed image or the information obtained from theantenna, a function of displaying the photographed image or theinformation obtained from the antenna on the display portion, and thelike. Note that the digital camera in FIG. 12B can have a variety offunctions without being limited to the above.

FIG. 12C illustrates a television set that can include a housing 9630, adisplay portion 9631, speakers 9633, operation key 9635, a connectionterminal 9636, and the like. The television set in FIG. 12C has afunction of converting an electric wave for television into an imagesignal, a function of converting an image signal into a signal suitablefor display, a function of converting the frame frequency of an imagesignal, and the like. Note that the television set in FIG. 12C can havea variety of functions without being limited to the above.

FIG. 12D illustrates a monitor for electronic computers (personalcomputers) (the monitor is also referred to as a PC monitor) that caninclude a housing 9630, a display portion 9631, and the like. As anexample, in the monitor in FIG. 12D, a window 9653 is displayed on thedisplay portion 9631. Note that FIG. 12D illustrates the window 9653displayed on the display portion 9631 for explanation; a symbol such asan icon or an image may be displayed. Since still images are oftendisplayed on the monitor for personal computers, the method for drivinga liquid crystal display device in the above-described embodiment ispreferably applied. Note that the monitor in FIG. 12D can have variousfunctions without being limited to the above.

FIG. 13A illustrates a computer that can include a housing 9630, adisplay portion 9631, a speaker 9633, operation keys 9635, a connectionterminal 9636, a pointing device 9681, an external connection port 9680,and the like. The computer in FIG. 13A can have a function of displayinga variety of information (e.g., a still image, a moving image, and atext image) on the display portion, a function of controlling processingby a variety of software (programs), a communication function such aswireless communication or wired communication, a function of beingconnected to various computer networks with the communication function,a function of transmitting or receiving a variety of data with thecommunication function, and the like. Note that the computer in FIG. 13Ais not limited to having these functions and can have a variety offunctions.

FIG. 13B illustrates a mobile phone that can include a housing 9630, adisplay portion 9631, a speaker 9633, operation keys 9635, a microphone9638, and the like. The mobile phone in FIG. 13B can have a function ofdisplaying a variety of information (e.g., a still image, a movingimage, and a text image) on the display portion; a function ofdisplaying a calendar, a date, the time, or the like on the displayportion; a function of operating or editing the information displayed onthe display portion; a function of controlling processing by variouskinds of software (programs); and the like. Note that the functions ofthe mobile phone in FIG. 13B are not limited to those described above,and the mobile phone can have various functions.

FIG. 13C illustrates an electronic device including electronic paper(also referred to as an eBook or an e-book reader) that can include ahousing 9630, a display portion 9631, operation keys 9632, and the like.The e-book reader in FIG. 13C can have a function of displaying avariety of information (e.g., a still image, a moving image, and a textimage) on the display portion; a function of displaying a calendar, adate, the time, and the like on the display portion; a function ofoperating or editing the information displayed on the display portion; afunction of controlling processing by various kinds of software(programs); and the like. Note that the e-book reader in FIG. 13C canhave a variety of functions without being limited to the abovefunctions. FIG. 13D illustrates another structure of an e-book reader.The e-book reader in FIG. 13D has a structure obtained by adding a solarbattery 9651 and a battery 9652 to the e-book reader in FIG. 13C. When areflective liquid crystal display device is used as the display portion9631, the e-book reader is expected to be used in a comparatively brightenvironment, in which case the structure in FIG. 13D is preferablebecause the solar battery 9651 can efficiently generate power and thebattery 9652 can efficiently charge power. Note that when a lithium ionbattery is used as the battery 9652, an advantage such as reduction insize can be obtained.

In the electronic device described in this embodiment, deterioration ofa displayed image can be reduced when a still image is displayed.

This embodiment can be implemented in appropriate combination with anyof the components described in the other embodiments.

This application is based on Japanese Patent Application serial No.2009-295608 filed with Japan Patent Office on Dec. 25, 2009, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

100: display panel, 101: pixel portion, 102: gate line, 102D: gate linedriver circuit, 103: signal line, 103D: signal line driver circuit, 104:pixel, 105: common electrode, 106: capacitor line, 106A: first capacitorline, 106B: second capacitor line, 107: terminal portion, 108: pixeltransistor, 109: liquid crystal element, 110: capacitor, 111: pixelelectrode, 112: counter electrode, 113: liquid crystal, 121: arrow, 122:arrow, 123: arrow, 301: display panel portion, 302: peripheral circuitportion, 303: image switching circuit, 304: display control circuit,305: holding signal generation circuit, 400: substrate, 401: gateelectrode layer, 402: gate insulating layer, 403: oxide semiconductorlayer, 405 a: source electrode layer, 405 b: drain electrode layer, 407:insulating layer, 409: protective insulating layer, 410: transistor,420: transistor, 427: insulating layer, 430: transistor, 440:transistor, 446 a: wiring layer, 446 b: wiring layer, 447: insulatinglayer, 501: first current source circuit, 502: first switch, 503: secondswitch, 504: second current source circuit, 505: third switch, 506:terminal, 507: switching terminal, 1400: display panel, 1401: pixelportion, 1402: gate line, 1403: signal line, 1404: pixel, 1405: commonelectrode, 1406: capacitor line, 1407: terminal portion, 1408: pixeltransistor, 1409: liquid crystal element, 1410: capacitor, 1411: pixelelectrode, 1412: counter electrode, 1413: liquid crystal, 1501: arrow,1502: arrow, 1503: arrow, 2600: TFT substrate, 2601: counter substrate,2602: sealant, 2603: pixel portion, 2604: display element, 2605:coloring layer, 2606: polarizing plate, 2607: polarizing plate, 2608:wiring circuit portion, 2609: flexible wiring board, 2610: cold cathodetube, 2611: reflective plate, 2612: circuit board, 2613: diffusionplate, 4001: first substrate, 4002: pixel portion, 4003: signal linedriver circuit, 4004: gate line driver circuit, 4005: sealant, 4006:second substrate, 4008: liquid crystal layer, 4010: thin filmtransistor, 4011: thin film transistor, 4013: liquid crystal element,4015: connection terminal electrode, 4016: terminal electrode, 4018:FPC, 4019: anisotropic conductive film, 4020: insulating layer, 4021:insulating layer, 4030: pixel electrode layer, 4031: counter electrodelayer, 4032: insulating layer, 4033: insulating layer, 4035: spacer,4040: conductive layer, 4041 a: insulating layer, 4041 b: insulatinglayer, 4042 a: insulating layer, 4042 b: insulating layer, 9630:housing, 9631: display portion, 9632: operation key, 9633: speaker,9635: operation key, 9636: connection terminal, 9638: microphone, 9651:solar battery, 9652: battery, 9653: window, 9672: recording mediumreading portion, 9676: shutter button, 9677: image receiving portion,9680: external connection port, 9681: pointing device

The invention claimed is:
 1. A display device comprising: a first pixelcomprising: a first transistor; a first liquid crystal element; and afirst capacitor; a second pixel comprising: a second transistor; asecond liquid crystal element; and a second capacitor; and a holdingsignal generation circuit, wherein each of the first transistor and thesecond transistor comprises an oxide semiconductor layer comprising achannel formation region, wherein one of a source and a drain of thefirst transistor is electrically connected to a pixel electrode of thefirst liquid crystal element, wherein a first electrode of the firstcapacitor is electrically connected to the pixel electrode of the firstliquid crystal element, wherein a second electrode of the firstcapacitor is electrically connected to a capacitor line, wherein one ofa source and a drain of the second transistor is electrically connectedto a pixel electrode of the second liquid crystal element, wherein afirst electrode of the second capacitor is electrically connected to thepixel electrode of the second liquid crystal element, wherein a secondelectrode of the second capacitor is electrically connected to thecapacitor line, wherein the holding signal generation circuit isconfigured to supply a constant voltage to the capacitor line during amoving image display, and wherein the holding signal generation circuitis configured to supply a holding signal to the capacitor line during astill image display such that a change of a voltage applied to the firstliquid crystal element and a change of a voltage applied to the secondliquid crystal element are reduced.
 2. The display device according toclaim 1, wherein a gate of the first transistor is electricallyconnected to a gate line, and wherein a gate of the second transistor iselectrically connected to the gate line.
 3. The display device accordingto claim 1, wherein the other of the source and the drain of the firsttransistor is electrically connected to a signal line, and wherein theother of the source and the drain of the second transistor iselectrically connected to the signal line.
 4. The display deviceaccording to claim 1, wherein a gate of the first transistor iselectrically connected to a first gate line, wherein a gate of thesecond transistor is electrically connected to a second gate line,wherein the other of the source and the drain of the first transistor iselectrically connected to a first signal line, and wherein the other ofthe source and the drain of the second transistor is electricallyconnected to a second signal line.
 5. The display device according toclaim 1, wherein an off-state current of the first transistor is equalto or lower than 1×10⁻¹⁷ A/μm.
 6. The display device according to claim1, wherein the holding signal generation circuit comprises a currentsource circuit, and wherein the current source circuit is electricallyconnected to the second electrode of the first capacitor through thecapacitor line.
 7. A display device comprising: a first pixelcomprising: a first transistor; a first liquid crystal element; and afirst capacitor; a second pixel comprising: a second transistor; asecond liquid crystal element; and a second capacitor; and a holdingsignal generation circuit, wherein each of the first transistor and thesecond transistor comprises an oxide semiconductor layer comprising achannel formation region, wherein one of a source and a drain of thefirst transistor is electrically connected to a pixel electrode of thefirst liquid crystal element, wherein a first electrode of the firstcapacitor is electrically connected to the pixel electrode of the firstliquid crystal element, wherein a second electrode of the firstcapacitor is electrically connected to a capacitor line, wherein one ofa source and a drain of the second transistor is electrically connectedto a pixel electrode of the second liquid crystal element, wherein afirst electrode of the second capacitor is electrically connected to thepixel electrode of the second liquid crystal element, wherein a secondelectrode of the second capacitor is electrically connected to thecapacitor line, wherein the holding signal generation circuit isconfigured to supply a constant voltage to the capacitor line during afirst frame period, wherein the holding signal generation circuit isconfigured to supply a holding signal to the capacitor line during asecond frame period such that a change of a voltage applied to the firstliquid crystal element and a change of a voltage applied to the secondliquid crystal element are reduced, and wherein the second frame periodis longer than the first frame period.
 8. The display device accordingto claim 7, wherein a gate of the first transistor is electricallyconnected to a gate line, and wherein a gate of the second transistor iselectrically connected to the gate line.
 9. The display device accordingto claim 7, wherein the other of the source and the drain of the firsttransistor is electrically connected to a signal line, and wherein theother of the source and the drain of the second transistor iselectrically connected to the signal line.
 10. The display deviceaccording to claim 7, wherein a gate of the first transistor iselectrically connected to a first gate line, wherein a gate of thesecond transistor is electrically connected to a second gate line,wherein the other of the source and the drain of the first transistor iselectrically connected to a first signal line, and wherein the other ofthe source and the drain of the second transistor is electricallyconnected to a second signal line.
 11. The display device according toclaim 7, wherein an off-state current of the first transistor is equalto or lower than 1×10⁻¹⁷ A/μm.
 12. The display device according to claim7, wherein the holding signal generation circuit comprises a currentsource circuit, and wherein the current source circuit is electricallyconnected to the second electrode of the first capacitor through thecapacitor line.